13 thoughts on “Advanced Space Propulsion”

  1. Would agree with everything he says with one important caveat. He is underestimating the significance of reusability in even our current chemical rockets. Even limiting ourselves just to that it would become feasible to have refueling stations in earth orbit on the Moon in the L5 area etc. That would greatly increase the effective range of even chemical rockets and (don’t think he mentioned) solid core NERVA like rockets. Also a BFG/Starship sent to earth orbit could with multiple trips send 100’s or even thousands of tons (eventually) of material/components/supplies into earth orbit that could be assembled into larger structures including more space-worthy nuclear powered interplanetary vehicles.

  2. I agree with the potential of beamed propulsion where you leave the propellant behind. However, Elon’s ITS presentation showed how in-orbit refueling can turn a chemical rocket into something with the capability normally attributed to NTRs. And since Hohmann transfers start on the opposite side of the destination, the travel times to the asteroid belt and even Jupiter are not terribly further than the Hohmann to Mars.

  3. I came to that same conclusion some time ago.

    The best place to generate the beamed power is at or inside Earth’s orbit. The main source of straightforwardly recoverable construction mass for space-based habitation and all other purposes is going to be the Asteroid Belt or beyond. So the future Solar System economy is going to be built on a base of energy exports to the Outer Solar System being the main industry of the Inner Solar System and of mass being the main export of the Outer Solar System to the Inner Solar System. Actual manufacturing will occur everywhere.

    1. “So the future Solar System economy is going to be built on a base of energy exports to the Outer Solar System being the main industry of the Inner Solar System and of mass being the main export of the Outer Solar System to the Inner Solar System.”

      At least until we have functional He3-De fusion drives like the “z-pinch”. Then with the access that would afford us to the Outer solar systems resources like the trillions of tons of He3 recoverable from the atmospheres of say Saturn, and (likely) Uranus & Neptune that might change the equation. Also no laser expert by any means but I thought Diffractional beam spreading of the laser beam from Earth orbit would greatly reduce your ability to transmit power over great distances.

      1. I think beamed energy for ship propulsion is likelier to be in the microwave/maser band than visible light/laser, but diffraction applies in either case.

        Diffraction spreading can be usefully limited via use of sufficiently large optics at the beam origination end. The late Dr. Robert Forward designed a laser beam power transmission scheme for interstellar ships that limited diffraction spreading usefully over distances of 10’s of light years via use of giant rotating Fresnel lenses in free space.

        The advent of practical fusion power would certainly influence the future Solar System economy, but would still depend on harvest and transport of matter on a continuing basis.
        For ships, especially military/exploration ships, operating in the Kuiper Belt and Oort Cloud, independent power of this type likely would displace beamed power as prime mover. For installations with orbits in free space, though – habs and industrial facilities – beamed power would still offer advantages.

  4. Yes, it’s the power supply, but more specifically, it is the radiator.

    All of the mentioned power cycles not only require a heat source, they require a “cold source”, that is, a way to reject heat. Even a solar cell needs to reject heat. I got into an argument on this point with one of the more smug participants on Slashdot, but I learned that a solar cell is a heat engine subject to the 2nd Law in my Electric Properties of Materials class. So there, argument by engineering class textbook!

    A space radiator, however, is not like the radiator in your brownstone apartment. That radiator not only radiates by the Stefan-Boltzmann T^4 law, but it also exchanges heat by conduction to neighboring air and convection of that air. You only have the fourth-power T radiation law in space, and this imposes a high temperature to move much heat past anything that isn’t an enormous surface.

    In the case of the solar cell, a solar panel has a large surface area to begin with, but a nuclear-thermal (or even thermo-electric) system may require a similar size for just the radiator. Yes, the fins on those thermoelectric generators aren’t that big, but they account for much of the bulk of these devices that generate only pitiful amounts of electric power.

    The problem even affects science fiction. One of the early models of the Discovery spacecraft in the movie 2001 had largish space radiators for the supposed nuclear-electric propulsion, but this model was ditched because it didn’t look cool. The scriptwriters didn’t want to explain why the spacecraft was all panels.

    What they ended up with makes some sense — a long pole separating the reactor compartment from the crew fits in with using distance and “shadow” shielding for radiation protection, but what those vertebrae-like thingies on the side of the pole was never explained. If they are tanks for LH2 reaction mass for nuclear thermal, they are way too small.

    There are outside-the-box ideas for space radiators. The radiator doesn’t need to be a solid structure, it could be a water or ice-pellet spray, but you need some way to collect and reuse this material so you don’t run out.

    1. Beamed power, by the way, may run up against the same limit.

      The way nuclear-thermal works is that the Carnot Cycle 2nd-Law heat rejection is to the reaction mass thrown out the back. If the beamed power is collected with a solar cell, you are still back to the radiative area limitation.

      1. But if it’s laser radiation, one can tune the band gap of the PV cell to that frequency and get significantly higher efficiency than with a broadband solar cell. Microwave conversion efficiency is inherently high, and rectennas have high surface area, but are light weight. But I would use plasma pulse propulsion with the laser, anyway.

        Jeff is absolutely right, and I have long had the same pet peeve about VASIMIR. However, ion main propulsion has been used to enable a mission that would have been impossible with chemical propulsion. The Dawn asteroid mission used a solar powered xenon ion main propulsion system to do its multiple asteroid visit mission, all using a Delta II to get it aloft. It didn’t make the mission faster. It just made it possible.

  5. It may be a little premature to write off fission power sources; I don’t think there’s been a very serious effort to build a mass-optimized reactor.

    If you like this sort of thing you might want a look at the computer game “Children of a Dead Earth” on Steam (alas, you missed the Black Friday sale, but…) which has limited but physics-realistic modeling. FWIW (and this is more useful as an upper bound than as anything feasible), the most advanced player built reactors produce about 60KW/Kg (weapons-grade uranium oxide cooled by liquid sodium), down to maybe half that once you include the radiators (acres of diamond-glass panels running at 2400K). Some sort of liquid-spray or phase-change radiators would likely be better but the game engine doesn’t support it – and a number of issues with the reactors that the game can’t/doesn’t model probably makes that insane power/radiation density optimistic at best. Still, that’s only 10x what’s required and it’s already handwaving some of the engineering issues, if not the fundamental thermodynamic ones.

    This also points out the issue with real torch drives (e.g. the Expanse’s Epstein drive); if you have crazy-high ISP and high thrust the waste heat will almost unaviodably bake your ship.

  6. I left a comment on that site, but it never posted so I thought I’d put it here for discussion: How about if we put up thousands of Aldrin Cyclers that provide beamed power to the manned ship?

    Each cycler would be small, mostly batteries with a small solar cell array and microwave power transmitter. It would just sit there most of the time, charging the battery. Then when a manned ship goes by it would transmit the stored power to for propulsion.

    It gets rid of the dock or die problem, but also allows the infrastructure to be developed slowly. If you want a larger payload or a faster flight, you can just launch more cyclers. It would be easy to make any level of redundancy that is desired. The major advantage is that you only accelerate the power supply once, of course.

    Any thoughts?

  7. In one of my old 1990s novels, an interstellar crossing is facilitated by accelerating the starship via magsail pushed by a solar-powered particle beam from well inside Mercury’s orbit. Deceleration at the target star was via “chromobraking.” This last bit was fun to write!

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